And yeah, that's the point he was going into a loveless marriage and wanting the love of his life. Hayato probably died young, given his appearance. He knows he's a coward (I understand the time period and honoring family tradition but he should have talked to Touma) and his regrets basically the kick start his soul reincarnating.
In tradition, if I looked correctly: Regrets and unresolved emotional issues from past lives are seen as karmic attachments that can continue to influence a person's present life and future rebirths. These attachments can create negative patterns and hinder progress on the path of spiritual development.
Oh I didn’t know the bit about karmic attachment, make sense. That’s interesting, thank you.
Honestly ? I hope Hayato died of old age, having a failed marriage and a miserable life crying every day over his lover’s death and being eaten by guilt. That’s what he deserves. And that girl that blackmailed him into marrying also deserve to be stuck in an unhappy unloving relationship.